Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within cells (see, e.g., Hardie and Hanks, The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif., 1995). Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases can be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these families (see, e.g., Hanks & Hunter, (1995), FASEB J. 9:576-596; Knighton et al, (1991), Science 253:407-414; Hiles et al, (1992), Cell 70:419-429; Kunz et al, (1993), Cell 73:585-596; Garcia-Bustos et al, (1994), EMBO J. 13:2352-2361).
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies, asthma, alzheimer's disease and hormone-related diseases. As a consequence, there has been substantial efforts in medicinal chemistry to find inhibitors of protein kinases for use as therapeutic agents.
Immunoreceptor tyrosine activation motif (ITAM)-mediated signaling has emerged as a primary event in signaling pathways responsible for human pathologies. ITAM-mediated signaling is responsible for relaying activation signals initiated at classical immune receptors such as T-cell receptors, B-cell receptors, Fc receptors in immune cells and at GPVI and FcγRIIa in platelets to downstream intracellular molecules such as Syk and ZAP-70 (Underhill, D. M and Goodridge, H. S., Trends Immunol., 28:66-73, 2007).
The binding of a ligand to an ITAM-containing receptor triggers signaling events which allows for the recruitment of proteins from a family of nonreceptor tyrosine kinases called the Src family. These kinases phosphorylate tyrosine residues within the ITAM sequence, a region with which the tandem SH2 domains on either Syk or ZAP-70 interact.
Syk, along with Zap-70, is a member of the Syk family of protein tyrosine kinases. The interaction of Syk or ZAP-70 with diphosphorylated ITAM sequences induces a conformation change in the kinases that allows for tyrosine phosphorylation of the kinase itself. Phosphorylated Syk family members activate a multitude of downstream signaling pathway proteins which include Src homology 2 (SH2) domain containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76), Linker of Activation of T-cells (LAT) and PLC (phospholipase C)γ2.
Human pathologies attributed to dysfunctional ITAM-mediated signaling include autoimmune diseases such as rheumatoid arthritis, systemic lupus, multiple sclerosis, hemolytic anemia, immune-thrombocytopenia purpura, and heparin-induced thrombocytopenia and arteriosclerosis. Interestingly, many of the above mentioned diseases are thought to occur through crosslinking of Fc receptors by antibodies which, via Syk, activate a signaling cascade in mast, basophil and other immune cells that result in the release of cell mediators responsible for inflammatory reactions. The release of mediators and the production of cytokines in IgE stimulation-dependent allergic and inflammatory reactions from mast cells and basophiles can be controlled by inhibiting the tyrosine kinase activity of Syk (Rossi, A. B. et al., J Allergy Clin Immunol, 118:749-755, 2006). In immune-thrombocytopenia, antibody bound platelets are cleared by the spleen by an Fc receptor/ITAM/Syk-mediated process (Crow, A. R. et al., Blood, 106:abstract 2165, 2005). Drug-induced thrombocytopenia, caused by heparin-platelet factor 4 immune complexes that activate platelet FcγRIIa, also involve Syk signaling downstream of receptor engagement (Reilly, M. P., Blood, 98:2442-2447, 2001).
Platelet agonists induce inside-out integrin signaling resulting in fibrinogen binding and platelet aggregation. This initiates outside-in signaling which produces further stimulation of platelets. Syk is activated during both phases of integrin signaling, and inhibition of Syk is shown to inhibit platelet adhesion to immobilized proteins (Law, D. A. et al., Blood, 93:2645-2652, 1999). Release of arachidonic acid and serotonin and platelet aggregation induced by collagen are markedly inhibited in platelets derived from Syk deficient mouse (Poole, A. et al., EMBO J., 16:2333-2341, 1997). Thus Syk inhibitors may also possess anticoagulation action.
Because of the role Syk plays in Ig-induced platelet activation, it is likely to be important in arteriosclerosis and restenosis. Arteriosclerosis is a class of diseases characterized by the thickening and hardening of the arterial walls of blood vessels. Although all blood vessels are susceptible to this serious degenerative condition, the aorta and the coronary arteries serving the heart are most often affected. Arteriosclerosis is of profound clinical importance since it can increase the risk of heart attacks, myocardial infarctions, strokes, and aneurysms.
The traditional treatment for arteriosclerosis includes vascular recanalization procedures for less-serious blockages and coronary bypass surgery for major blockages. A serious shortcoming of intravascular procedures is that, in a significant number of treated individuals, some or all of the treated vessels restenose (i.e., re-narrow). For example, restenosis of an atherosclerotic coronary artery after PTCA (Percutaeous Transluminal Coronary Angioplasty) occurs in 10-50% of patients undergoing this procedure and subsequently requires either further angioplasty or a coronary artery bypass graft. Furthermore, restenosis of an atherosclerotic coronary artery after stenting occurs in 10-20% of patients undergoing this procedure and subsequently requires repeat treatments to maintain adequate blood flow through the affected artery. Restenosis generally occurs in a relatively brief time period, e.g., roughly less than six months, after treatment.
While the exact hormonal and cellular processes promoting restenosis have not been determined, restenosis is thought to be due in part to mechanical injury to the walls of the blood vessels caused by the balloon catheter or other intravascular device. For example, the process of PTCA, in addition to opening the obstructed artery, also injures resident coronary arterial smooth muscle cells (SMCs). In response to this injury, adhering platelets, infiltrating macrophages, leukocytes, or the smooth muscle cells themselves release cell-derived growth factors such as platelet-derived growth factor (PDGF), with subsequent proliferation and migration of medial SMCs through the internal elastic lamina to the area of the vessel intima. Further proliferation and hyperplasia of intimal SMCs and, most significantly, production of large amounts of extracellular matrix over a period of three to six months results in the filling in and narrowing of the vascular space sufficient to significantly obstruct blood flow.
In addition to the role Syk plays in Ig-induced platelet activations, Syk plays a very important role in collagen-mediated signaling. The primary adhesive protein responsible for platelet adhesion and activation is collagen. Collagen is a filamentous protein contained within the fibrotic caps of atheromas which becomes exposed to blood during plaque rupture. Collagen functions initially by binding von Willebrand factor which tethers platelets through binding platelet membrane GPIb. Collagen functions secondarily by engaging the two collagen receptors on platelets, GPVI and integrin α2β1.
GPVI exists in platelet membranes as a complex with FcRγ, an interaction required for the expression of GPVI. Activation of FcγRIIa on platelets results in platelet shape change, secretion and thrombosis. Signaling by the GPVI/FcRγ complex is initiated by tyrosine phosphorylation of the ITAM domain of FCRγ followed by the recruitment of Syk. Activation of GPVI leads to induction of multiple platelet functions including: activation of integrins α2β1 to achieve firm platelet adhesion, and GP IIb-IIIa which mediates platelet aggregation and thrombosis growth; platelet secretion, allowing for the delivery of inflammatory proteins such as CD40L, RANTES and TGFβ to the vessel wall; and the expression of P-selectin which allows for the recruitment of leukocytes. Therefore, it is believed that Syk inhibitors can inhibit thrombotic events mediated by platelet adhesion, activation and aggregation.
It has been reported that the tyrosine phosphorylation of intracellular protein (activation) induced by stimulation of a receptor for IgG antibody, FcγR, and the phagocytosis mediated by FcγR are considerably inhibited in macrophages derived from Syk deficient mouse (Crowley, M. T. et al, J. Exp. Med., 186:1027-1039, 1997). This suggests that Syk has a markedly important role in the FcγR-mediated phagocytosis of macrophages.
It has also been reported that an antisense oligonucleotide of Syk suppresses the apoptosis inhibition of eosinophils induced by GM-CSF (Yousefi, S. et al, J. E. Med., 183:1407-1414, 1996), showing that Syk is essential for the life extending signal of eosinophils caused by GM-CSF and the like. Since life extension of eosinophils is closely related to the transition of diseases into a chronic state in allergic disorders, such as asthma, Syk inhibitors can also serve as therapeutic agents for chronic eosinophilic inflammation.
Syk is important for the activation of B-cells via a B-cell antigen receptor and is involved in the phosphatidylinositol metabolism and increase in the intracellular calcium concentration caused by the antigen receptor stimulation (Hutchcroft, J E. et al, J. Biol. Chem., 267:8613-8619, 1992; and Takata, M. et al, EMBO J., 13:1341-1349, 1994). Thus, Syk inhibitors may be used to control the function of B-cells and are, therefore, expected to serve as therapeutic agents for antibody-related diseases.
Syk binds to a T-cell antigen receptor, quickly undergoes tyrosine phosphorylation through crosslinking of the receptor and synergistically acts upon intracellular signals mediated by Src tyrosine kinases such as Lck (Couture, C. et al, Proc. Natl. Acad. Sci. USA, 91:5301-5305, 1994; and Couture, C. et al, Mol. Cell. Biol., 14:5249-5258, 1994). Syk is present in mature T-cell populations, such as intraepithelial γδ T-cells and näive αβ T-cells, and has been reported to be capable of phosphorylation of multiple components of the TCR signaling cascade (Latour, S. et. al, Mol Cell Biol., 17:4434-4441, 1997). As a consequence, Syk inhibitors may serve as agents for inhibiting cellular immunity mediated by T-cell antigen receptor.
Recent comparative genomic hybridization studies have identified Syk as another gene important in the pathogenesis of Mantle Cell Lymphoma (MCL) (Chen, R. et al. Journal of Clinical Oncology, 2007 ASCO Annual Meeting Proceedings (Post-Meeting Edition). Vol 25, No 18S (June 20 Supplement), 2007: 8056). MCL represents 5-10% of all non-Hodgkins lymphomas and it is a difficult form of lymphoma to treat. It has the worst prognosis among the B cell lymphomas with median survival of three years. It has been reported that Syk is overexpressed in MCL (Rinaldi, A, et. al, Br. J. Haematol, 2006; 132:303-316) and that Syk mediates mTOR (mammalian target of Rapamycin) survival signals in follicular, mantel cell, Burkitt's, and diffuse large B-cell non-Hodgkin's lymphomas (Leseux, L., et. al, Blood, 2006; 108:4156-4162).
Several lines of evidence suggest that many B-cell lymphomas depend upon B-cell receptor (BCR)-mediated survival signals. BCR signaling induces receptor oligomerization and phosphorylation of Igα and β immunoreceptor tyrosine-based activated motifs by SRC family kinases. ITAM phosphorylation results in the recruitment and activation of Syk that initiates downstream events and amplifies the original BCR signal. Given the role of tonic BCR signaling in normal B cell and Syk-dependent survival of non-Hodgkins lymphoma cell lines in vitro (Chen, L., et. al, Blood, 2006; 108:3428-3433), Syk inhibition is a promising rational treatment target for certain B-cell lymphomas and chronic lymphocytic leukemia (CLL) (Stefania Gobessi, Luca Laurenti, Pablo Longo, Laura Carsetti, Giuseppe Leone, Dimitar G. Efremov, Constitutive activation of the protein tyrosine kinase Syk in Chronic Lymphocytic Leukemia B-cells, Blood, 2007, 110, Abstract 1123). Recent data shows that administration of a multikinase inhibitor which inhibits Syk, may have significant clinical activity in CLL patients (Friedberg J W et al, Blood 2010; 115(13)).
The oncogenic potential of the spleen tyrosine kinase (Syk) has been described in a number of different settings. Clinically, Syk over-expression is reported in Mantle Cell Lymphoma (Rinaldi, A, et. al, Br. J. Haematol, 2006; 132:303-316) and the TEL-Syk fusion protein (Translocated ETS Leukemia) generated by a chromosomal translocation (t(9;12)(q22;p12)) leads to increased Syk activity and is associated with myelodysplastic syndrome (Kuno, Y., et. al, Blood, 2001; 97:1050-1055). Leukemia is induced in mice by adoptively transferring bone marrow cells that express human TEL-Syk (Wossning, T., JEM, 2006; 203:2829-2840). Further, in mouse primary bone marrow cells, over-expression of Syk results in IL-7 independent growth in culture (Wossning, T., et. al, JEM, 2006; 203:2829-2840). Additional recent studies also suggest that Syk-dependant survival signals may play a role in B-cell malignancies, including DLBCL (Diffuse Large B-Cell Lymphoma), mantle cell lymphoma and follicular lymphoma (Gururajan, Jennings et al. 2006; Irish, Czerwinski et al. J Immunol 176(10): 5715-9 (2006). Given the role of tonic BCR signaling in normal B cells and Syk-dependent survival of NHL cell lines in vitro, the specific inhibition of Syk may prove promising for the treatment of certain B-cell lymphomas.
Interestingly, Syk signaling appears to be required for B-cell development and survival in humans and mouse. Inducible loss of the B-cell receptor (Lam, K., et. al, Cell, 1997; 90:1073-1083) or Igα (Kraus, M., et. al, Cell, 2004; 117:787-800) results in loss of peripheral B-cells in mice. Over-expression of the protein tyrosine phosphatase PTP-RO, which is known to negatively regulate Syk activity, inhibits proliferation and induces apoptosis in cell lines derived from non-Hodgkin's lymphomas (Chen, L., et. al, Blood, 2006; 108:3428-3433). Finally, B-cell lymphomas rarely exhibit loss of BCR expression, and anti-idiotype therapy rarely leads to resistance (Kuppers, R. Nat Rev Cancer, 2005; 5:251-262).
Engagement of the antigen-specific B cell receptor (BCR) activates multiple signaling pathways that ultimately regulate the cells activation status, promoting survival and clonal expansion. Signaling through the BCR is made possible by its association with two other members of the immunoglobulin super-family; Igα and Igβ, each bearing an immuno-tyrosine based activation motif (ITAM) (Jumaa, Hendriks et al. Annu Rev Immunol 23: 415-45 (2005). The ITAM domain is directly phosphorylated by Src family kinases in response to BCR engagement. The spleen tyrosine kinase (Syk) docks with and phosphorylates the ITAM, a process that enhances its kinase activity, resulting in Syk autophosphorylation and tyrosine phosphorylation of multiple downstream substrates (Rolli, Gallwitz et al. Mol Cell 10(5): 1057-69 (2002). This signaling pathway is active in B cells beginning at the transition from pro- to pre-B cell stage of development, when the newly formed pre-BCR is expressed. In fact, B cell development arrests at the pro-B cell stage in Syk knockout mice (Cheng, Rowley et al. 1995; Turner, Mee et al. Nature 378(6554): 303-6 (1995). Inducible loss of the B cell receptor (Lam, Kuhn et al. Cell 90(6): 1073-83 (1997) or Igα (Kraus, Alimzhanov et al. Cell 117(6): 787-800 (2004) results in loss of peripheral B cells in mice. Human B cells also appear to require Syk for proliferation and survival. Over-expression of the protein tyrosine phosphatase PTP-RO, a negative regulator of Syk activity, inhibits proliferation and induces apoptosis in cell lines derived from non-Hodgkin's lymphomas (NHL) (Chen, Juszczynski et al. Blood 108(10): 3428-33 (2006). Knock down of Syk by siRNA in the NHL line SUDHL-4 led to a block in the G1/S transition of the cell cycle (Gururajan, Dasu et al. J Immunol 178(1): 111-21 (2007). Together, these data suggest that Syk signaling is required for the development, proliferation, and even survival of human and mouse B cells.
Recently, R406 (Rigel Pharmaceuticals) was reported to inhibit ITAM signaling in response to various stimuli, including FcεR1 and BCR induced Syk activation (Braselmann, Taylor et al. J Pharmacol Exp Ther 319(3): 998-1008 (2006). Interestingly, this ATP-competitive inhibitor of Syk was also active against Flt3, cKit, and JAK kinases, but not against Src kinsase (Braselmann, Taylor et al. 2006). Activating mutations to Flt3 are associated with AML (Acute Myeloid Leukemia) and inhibition of this kinase is currently under clinical development (Burnett and Knapper Hematology Am Soc Hematol Educ Program 2007: 429-34 (2007). Over-activation of the tyrosine kinase cKit is also associated with hematologic malignancies, and a target for cancer therapy (Heinrich, Griffith et al. Blood 96(3): 925-32 (2000). Similarly, JAK3 signaling is implicated in leukemias and lymphomas, and is currently exploited as a potential therapeutic target (Heinrich, Griffith et al. 2000). Importantly, the multikinase inhibitory activity of R406 attenuates BCR signaling in lymphoma cell lines and primary human lymphoma samples, resulting in apoptosis of the former (Chen, Monti et al. Blood 111(4): 2230-7 (2008). Further, a phase II clinical trial reported favorable results by this compound in refractory NHL and chronic lymphocytic leukemia (Friedberg J W et al, Blood 2010; 115(13)). Although the precise mechanism of action is unclear for R406, the data suggest that inhibition of kinases that mediate survival signaling in lymphocytes is clinically beneficial.
Additional recent studies also suggest that Syk-dependant survival signals may play a role in B-cell malignancies, including DLBCL, mantle cell lymphoma and follicular lymphoma (see e.g., S. Linfengshen et al. Blood, February 2008; 111: 2230-2237; J. M. Irish et al. Blood, 2006; 108: 3135-3142; A. Renaldi et al. Brit J. Haematology, 2006; 132: 303-316; M. Guruoajan et al. J. Immunol, 2006; 176: 5715-5719; L. Laseux et al. Blood, 2006; 108: 4156-4162.
While progress has been made in this field, there remains a need in the art for compounds that inhibit Syk kinase, as well as for methods for treating conditions in a patient, such as restenosis, and/or inflammation that can benefit from such inhibition. Moreover, the availability of compounds that selectively inhibit one of these kinases as compared to other kinases would also be desirable. The present invention satisfies this and other needs.